1. Field of the Invention
The present invention relates to a wiring board having a through hole and/or a non-through hole in which a metal wiring is formed. The metal wiring is made from spherical particles having a sphericity of about −10% to +10%. The spherical particles of the present invention can have a composite structure. The present invention also relates to a method for producing the wiring board.
2. Description of the Related Art
In order to form a metal wiring inside a through hole or a non-through hole, it was conventional to fill a conductive paste into a through hole or a non-through hole. Japanese Unexamined Patent Publication No. 11-298138 discloses a method for filling a viscosity paste into a through hole or a non-through hole for preparing a multilayer circuit board.
However, the invention disclosed by Japanese Unexamined Patent Publication No. 11-298138 can form a conductive wiring inside a hole whose diameter is as small as 50 μm, and whose aspect ratio (the ration between the length of the hole and the diameter of the hole) is as low as 2 or 3. Even if the conductive wiring can be formed by the conventional method, but since the conductive paste includes a resin, the conductivity of the wiring is not good. The conventional method can not assuredly form a metal wiring in the fine hole because there may exist non-filled portion inside the hole. Therefore, a novel metal material and production method are required for forming a metal wiring having a superior conductivity.
Meanwhile, micrometer-size particles of various metals, alloys, metal oxides, metal nitrides, metal silicides, or mixtures of these materials, having a spherical shape, are disclosed in U.S. Pat. No. 6,808,568. The micrometer-size particles have a particle size of 1-300 μm.
Further process can be applied to the micrometer-size particles into nanometer-size particles. The applicants of the present invention filed a U.S. provisional application No. 60/836,407, disclosing a nanometer-size spherical particles and the method for producing the particles. According to the method disclosed in the U.S. provisional application No. 60/836,407, spherical particles of various metals or alloys can be produced at a high yield. The sphericity of the nanometer-size particles can be about −10% to +10%.